The east-west division in Ukraine predates Soviet times.
Many parts of western Ukraine were absorbed into different states and empires over the years, and only in relatively recent times became part of what is known as 'Ukraine'.
'The history of Western Ukraine is closely associated with the history of the following lands:
Easternmost Bukovina, historical region of Central Europe in official use since 1775, controlled by Kingdom of Romania after World War I and mostly ceded to the USSR by the Paris Peace Treaties, 1947.
Eastern Galicia (Ukrainian: Halychyna), once a small kingdom with Lodomeria (1914), province of the Austrian Empire until the dissolution of Austria-Hungary in 1918. See also: crownland of the Kingdom of Galicia and Lodomeria
Red Ruthenia since medieval times in the area known today as Eastern Galicia.
West Ukrainian People's Republic declared in late 1918 until early 1919 and claiming half of Galicia with mostly Polish city dwellers (historical sense).
Carpatho-Ukraine region within Czechoslovakia (1939) under Hungarian control until the Nazi occupation of Hungary in 1944.
General Government of Galicia and Bukovina captured from Austria-Hungary during World War I.
?inutul Suceava (Kingdom of Romania)
Volhynia, historic region straddling Poland, Ukraine, and Belarus to the north. The alternate name for the region today is Lodomeria after the city of Volodymyr-Volynsky. See also: Polish unofficial term Kresy (Borderlands, 1918–1939) that includes the West Belarus as well as Volhynia.
Zakarpattia or Carpathian Ruthenia presently in the Zakarpattia Oblast of western Ukraine.'
There have always been alternative points of view to the one promoted by the OUN(b), that there is such a thing as Ukraine, that Ukraine is for Ukrainians, and that the Ukrainian language is the language of Ukraine. These points of view have not just been held by ethnic minorities within Ukrainian territory but by ethnic Ukrainians. The OUN(b) branded such views treason and actively silenced Ukrainian opposition either by murder or by intimidation. Only when the OUN(b) dominated among the Ukrainian population in both Volhynia and Galicia could it begin its campaign to wipe out Poles living there.
There have always been large groups of non-Ukrainians living in the current day Ukraine. There are fewer non-Ukrainians now living in regions currently in western Ukraine than there were before the end of WW2 however.
In the muddled state of affairs in eastern Europe following the collapse of the Hapsburg Empire, Imperial Russia and Imperial Germany, Curzon attempted to allocate territories to newly emerging states and to draw borders that were acceptable to all parties concerned. One guiding principle used by Lord Curzon was use of previous borders that Britain had recognised (for instance the 1797 border between Poland and Imperial Russia), and another factor that was taken into account was cultural and linguistic aspects of an area's makeup (for instance in the matter of which country Lvi/Lvov should belong to - it ended up in Poland). Galicia and Volhynia went to interwar Poland.
Mass murder and forced resettlement of various ethnic groups by the invading Nazis and their local associates and henchmen changed the ethnic character of many of the regions along the old Curzon line, both east and west of it. Most notably, the Jewish population was wiped out. Galicia and Volhynia, where the OUN(b) murdered up to 70,000 Poles among other targets, saw their Polish populations greatly reduced.
In an ironic twist, both regions became part of the post WW2 Ukrainian SSR and its Ukrainian successor state thanks to the Red Army.